With the use of hetero-, allo-, auto-, and hybridoma antibody, we plan to isolate and purify subsets of human T cells with the view of ascertaining whether or not individually or in reconstituted pools, they mediate a variety of immunologic activities in vitro including the response to antigens, mitogen, cytotoxic functions, mediator production, regulatory functions such as T-T, T-B, and T-Null interactions and the genetic control of such responses. These studies should provide an understanding of the cellular mechanisms and interactions involved in the generation of the human immune response and give insights into the T cell circuit in man which controls both the induction and suppression of a variety of immune responses. These regulatory functions have a profound effect on the host's response to invading organisms as well as to his own tissues and tumors.